Method and arrangement for limiting interferences in radio receiving apparatus



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METHOD AND ARRANGEMENT. FOR LIMITING INTERFERENCES IN RADIO RECEIVINGAPPARATUS Filed Jan. 3l, 1940 4 SheetSfShvee'b 2 JMJ looo zooo ooo n4o'oo sooo Jan. 6, `1942. Q DALLos y METHOD AND ARRANGEMENT FOR LINITINGINTERFERENCES IN RADIO RECEIVING APPARATUS Filed Jan. 3l, 1940 4Sheets-Sheet lE5 je @jalea/ s Arronuley Jan. 6, 1942.

I ALLos 2,269,011 METHOD AND ARRANGEMENT FOR LIMITING INTERFERENGES vINRADIO RECEIVING APPARATUS -Filed Jan. 31, 1940 .,4 Sheets-Sheet 4 SIW nl l gp l Ll.

yMmZ'E/z feo/7e v y Afm 5m LT-TQANEY Patented Jan. 6, 1942 METHOD ANDARRANGEMENT FOR LIlYIIT- ING INTERFERENCES IN RADIO RECEIV- INGAPPARATUS George Dallos, Budapest, Hungary, assignor to- MagyarWolframlmpa Gyr Kremenezky Jnos Rszvnytrsasg, Budapest, Hungary, aHungarian company Application January 31, 1940, Serial No. 316,469 InHungary October 28, 1938 8 Claims.

It is known that the oscillatory circuits of a radio receiver aredisturbed by an aperiodic voltage impulse, that is to say an interferingimpulse, which is received by the receiver. The Fourier analysis of theresulting interference curve shows that the voltage produced by theinterfering impulse throughout the whole of the usual reception range`of approximately 9 kilocycles contains components which have approxi'-mately equal amplitudes, so that the interference forms as it were acontinuous spectrum independently of the duration of the interferingimpulse. On the other hand, the frequency distribution of the modulationis discontinuous, that is to say the places where modulation occurs andthe amplitudes of the frequencies contained in the modulation aredetermined by the sound to be transmitted. Itmay happen, for example inthe case of awomans voice, that the higher sound frequencies predominateand that the lower frequencies are almost entirely absent. Thisdistribution of frequency may take place in the most varied ways inthevsound to be transmitted. Y f

It has previously been proposed to subdivide the receiving band receivedinto a plurality of channels and to eliminate interference by amplitudelimitation in the individual channels and especially in those comprisingthe high sound frequencies. The effect of the interference is notlimited by this method to any substantially greater extent than by thoseknown methods which prevent a sudden increase in sound frequencyamplitude beyond the maximum amplitude corresponding, for example, to100% modulation. The reason for Athis is that, on the one hand in eachof the channels in which an amplitude limitation takes place theadjustment of the biassing voltage of the limiting device isindeterminate because this vdepends on the average value of the highfrequency carrier wave, vand on'the other hand elimination ofinterference is effected only in a part of the frequency band, noaccount being paid to the continuous course of the interferencespectrum.

. It has also been proposed to eliminate interference in a receiver independence on the degree of modulation. This method, in addition to thedifficulty of carrying it into effect, has the disadvantage that thelimiting level is determined by the greatest amplitude existing in thesound frequency band and is independent of the frequency.

vBy the method of they present invention the are obviated and at thesame time excellent results in the elimination of interference are obvtained, since the interference is suppressed to the same extent asuseful signals are received in the receiver. In this way, the differencebetween the interference and the signal, namely that the spectrum of theinterference is continuous and that of the signal is discontinuous, isfully utilized.

According to the present invention, there are used for the furthertransmission of the instantaneous signal frequencies only those channelsof the receiving band in which a component of the vinstantaneous signaloccurs and the magnitude of the voltages which can be passed through theindividual channels is'made dependent on the amplitude of the signalfrequencies in the channel in question.

In one method of carrying out the invention aseparate amplitude.limiting device is connected in each individual channel in the low.frequency part of the receiving apparatus, and the limiting voltage forthe amplitude limiting device is determined by the component of thesignal frequencies in the channel' in question.` For pro-A ducing thelimiting voltage for the amplitude limiting device the component of thesignal frequencies inthe channel can be supplied to a rectifying stagewhich is connected through a filter circuit with the lamplitude limitingdevice. Inl order that the invention may be readily understood andeasily carried into effect it will now be described by Way of examplewith reference to the accompanying drawings, in which Figures 13 eachrepresent a sound diagram divided into a plurality of channels.

c Figure 4` is a diagrammatic representation of an arrangement having aplurality of channels which is suitable for carrying out the method ofthe invention, and

, Figure 5 is a circuit diagram of one of the channels of thearrangement shown in Figure 4.

In Figures 13 the frequencies in cycles per second are plotted asabscissae and the sound frequency energy E in any unit is plotted asordinates.

Referring to the response diagram illustrated in Figure 1, thefrequency` band which extends up to 5000 cycles per second is dividedinto eleven channels designated I-Il ,1 and the sound-frequency energyin eachvchannel at the moment of investigation is indicated in eachchannel by a thick vertical line. The curve gl represents thedistribution of theinterference components disadvantages of theabove-mentioned methods in each of the channels.` In the diagram ofFigure 1 there is no signal component in the channels 2, 5, 6 and whileon the other hand signal components exist in the channels 3, 4, 1, 8, 9and |0. For the transmission of this sound it is therefore sufcient ifthe last mentioned channels are allowed to function and the formerchannels are prevented from being effective. It

follows from this that only the interference com-V ponents which arepresent in the channels 3, 4, 1, 8, 9 and |U are further transmittedwhile those in the channels l, 2, 5, 6 and Il are not transmittedfurther. If now there is connected in each of the channels 3, 4, 1, 8, 9and I9 an amplitude limiting device the limiting action of which is madedependent on the energy of the sound frequency components occurring inthe individual channels, then as far as the area a, b,

c, d, bounded by the curve g, which represents the amount ofinterference, is concerned, only components which fall Within the fieldof the diagram which is not section-lined and which are insignificantand dov not interfere with the reception.

Figure 2 illustrates by way of example a further sound diagram. In thiscase, at the moment of making the investigation, the band is entirelycut off above a frequency of 2,300 cycles per second, so that theinterfering components represented by the curve g2 which fall within theband above that frequency are entirely eliminated. Similarly as inFigure l, in this case only the interfering components below the line k2and falling in the field which is not sectionlined are furthertransmitted.

The diagram of Figure 3 illustrates the theoretical case in which thereceiving band is subdivided into a very great number of' channels andthe reduction of interference is carried out in each channel. As may beseen from the diagram, it is possible by the method of the invention toreduce the interference to such an extent that it exactly follows thecourse of the sound diagram as is illustrated by the curve k3.

A diagram showing an arrangement suitable for carrying out the method ofthe invention is illustrated in Figure 4. In this figure the part of theapparatus up to the low frequency amplifier is indicated by the numeral20. The references 2|-28 designate low frequency band pass filters bymeans of which the receiving band is split up into eight channels. Thereceiving circuits of the respective channels are represented by therectangles indicating the filters. The nlters are connected respectivelyto amplitude limiters 3|-38 and these in turn are connected respectivelyto rectiers lll- 48 and the correspondingly filtered limiting voltagestherefrom are conducted through the leads 5|-58 to the amplitudelimiters. The separate channels are combined in the amplifier 60 whichis connected through a second amplifier 6| to a loud speaker 62.Obviously the signal components belonging to the individual channels canbe combined in any desired combination and may in some cases beconducted to separate loud speakers.

Figure 5 represents by way of 'example the circuit diagram of one of thechannels. The terminals 1| and 12 are connected to the rst stage of thelow frequency part of the apparatus. The signal components to betransmitted by the channel are separated from the complete low frequencysignal occurring at the terminals 1| and 12 by the filter 13. IIThesignal components then pass through a condenser 14 to the control gridof a limiting valve 16 which is constructed as a pentode and is providedwith a grid resistance 15. The valve 16 is connected in parallel (from adirect point of view) with a regulating valve 'l1 which is constructedas a triode and receives its anode voltage through a resistance 88.Between the anode lead and the earth connection of the valve 11 areinterconnected potentiometer resistances 18 and 19 which are connectedin series. The vconnecting point between the two resistances 18 and, 19is connected to the primary winding of a transformer 8| the other end ofwhich is connected to the anode of the valve 16. The secondary windingof the transformer 8| is connected to an amplifying valve 82 in theanode circuit of which is connected a condenser 84 as well as a choke 83serving as a load impedance. The output terminals are indicated by 85and 86. From this anode circuit is branched off through a condenser 81the circuit of a diode 88 which is connected by way of the filter 98 and9| to the regulating valve 11. The diode 88 has a loading resistance 89the tapping point of which is connected through the filter 92 and 93 tothe grid leak resistance 15 of the limiting valve 16. v

In order to be better able to understand the invention the operation ofthe circuit illustrated will be shortly described. If the anode voltageof the valve 16 is less than the voltage on its screen grid, asaturation characteristic is obtained of which the section of positivesteepness enabling an undistorted amplification, varies according to thevoltage on the positive electrodes of the valve. 'Ihe screen gridvoltage and the anode voltage of the valve 16 are regulated by thetriode 11. If the current flowing through the triode 11 increases, thevoltage drop at the resistance becomes greater, the consequence of whichis that screen grid and the anode of the Valve 16 receive a smallervoltage, whereby the limiting level is reduced. If, however, the currentthrough the triode 11 falls the voltage on both the aforesaid electrodesincreases as does also the limiting level. The oscillations limited inthis way pass from the Valve 16 through the transformer 8| to theamplifying valve 82 and thence to the output terminals and 86.

The regulation of the bias voltage on the control grid of the regulatingvalve 11 and therefore of the strength of the current flowing throughthe triode in order that it may be a function of the sound frequencyvoltage, takes place in the following manner.- The sound frequencyalternating voltage amplified in the amplifier 82 is rectified by thediode 88, the voltage at the resistance 89 of which is proportional tothe instantaneous voltage occurring in the channel. This voltage ispassed through the filter 98, 9| and is then applied as a regulatingvoltage to the control grid of the valve 11. rIf the amplitude of theregulating voltage falls, that is to say if the negative bias voltage onthe regulating valve 11 becomes less, the anode current flowing throughthis valve increases, so that, as stated previously, the limitingV levelof the .-.velve 1.6 adjusts itself to. af ,lower .value corresponding fto )the 1-emp1itudefto. .be transmitted- Ifthe regulating .-yoltage eincreases, the converse is the case andthe limiting "level -of the valve16 is 'raised.L Thus theqlevel of limiting amplitude follows accuratelythe Vvamplitudes to. be transmitted. If the amplitude to be transmittedfalls to z'ero, that is tosay ify there yis no signal component in thechannel-in question, Athe transmission in the. channel is practicallyinterrupted or at least reduced to a minimum. The biassing .voltage bywhich theworking pointof the valve 16 fis determined is also takenfromfthe resistance 89 and passed through filter 92 and 93 andtheresistance l5 tothecontrol grid of the limiting A valve '16.

The filters 90, 9| and 92, 93 should be so dimensioned that their timefactor is greater than the period of oscillation of the lowest signalfrequency occurring in the channel in question.

The circuits of al1 the channels can be of the kind described. However,other amplitude limiting circuits can be used without affecting theinvention in any Way. The limiting level in all the channels will riseor fall in accordance with the larger or smaller alternating voltages inthe individual channels.- The effect illustrated in Figures 1-3 istherefore produced and the interference is practically entirelyeliminated.

The same effect can also be obtained by dividing the high orintermediate frequency part of the receiving apparatus into a number ofchannels and rectifying the oscillations in the individual channels. Inthis case, however, the limiting voltage for the amplitude limitersconnected in the individual channels must consist of two parts of whichone is proportional to the average value of the high or intermediatefrequency arriving in the bands in question, that is to say a slowlyvarying voltage While the other, on the other hand, is proportional tothe amplitude of the modulation frequency in the band in question, thatis to say is a rapidly varying voltage.

The highest permi-ssible number of channels is in any case such that theindividual channels can follow the oscillations of the signal componentto the full extent.

For carrying the invention into effect any desired and known device orcircuit which effects a limiting effect can be used and it is onlynecessary to take care that the working characteristic of the limitingdevice possesses sharp lower and upper bends. l

The circuits in accordance with the invention can be combined withautomatic band width regulators in such a way that the Width of theresonance curve and the place of the receiving band is made dependent onthe modulation, that is to say that when the higher sound frequencycomponents are absent in the modulation, the band Width is automaticalyreduced. Also any known tuning device can be used on the two receivingside bands which functions in such a way that if many high soundfrequency components but no low sound frequency components are presentin the receiving band, the receiving band is shifted from the carrier tothe right or to the left. There is thus obtained a band Width regulationoperating as a function of the modulation Which results in betterreception. The band Width regulator or automatic tuning device describedis employed in the high or intermediate frequency part of the apparatus.

What I claim is:

1. In the method of limiting interference in radio reception iny whichthe band received-fis split up into a plurality of sub-bands--they im-Vthe other sub-bandsto a value dependent on the maximum amplitude of theinstantaneous signal frequency therein to thereby permit transmission Yineach sub-band in Which an instantaneous sigynal frequency occurs ofonly those interfering components having' amplitudes not substantiallygreater than the. maximum amplitude of the instantaneoussignalmfrequency therein.

2.,A In a radio receiving system includingv a plurality offilter-provided channels for dividing the total frequency band'of thesystem into as `many subbands, and .wherein only those channels areactive at leach instant in which components of received signalfrequencies occur, the combination with each rchannel of an amplitudelimiter, means connected to each of said amplitude limiters to receiveaudio frequency alternating voltages, means for transforming voltagesreceived by each of said first-mentioned means into controlling voltagesfor the amplitude limiter connected thereto, and means for applying saidcontrolling voltages to the respective amplitude limiters so as to limitthe maximum voltage passing through the particular channel.

3. A system as claimed in claim 2 in which each filter circuit has atime factor greater than the period of oscillation of the lowest signalfrequency occurring in the'particular circuit.

4. A system as claimed in claim 2 in which each amplitude limitercomprises a multi-grid valve having a screen grid and an anode .which isat a less positive potential than the screen grid.

5. In a radio receiving system including in a stage higher than a lowfrequency stage, a plurality of filter-provided channels for dividingthe total frequency band of the system in as many subbands, and whereinonly those channels are active at each instant, in which components ofreceived signal frequencies occur, the combination, with each channel,of an amplitude limiter, means connected to each amplitude limiter forrectifying the oscillations in the particular channel, a two-part meansfor applying controlling voltage to said amplitude limiter, one of saidparts being adapted to supply one part of the controlling voltage inproportion to the average amplitude 'of the frequency in the particularchannel, the other one beingl adapted to supply yanother part of saidvoltage in proportion to the amplitude of the modulationfrequency in theparticular channel.

6. In the method of limiting interference in radio reception in whichthe band received is split up into a plurality of sub-bands, theimprovement which consists in substantially preventing energy from beingtransmitted in those subbands in which no component of the instantaneoussignal frequency occurs, and simultaneously limiting the maximum voltagein each of the other sub-bands to a value dependenton the maximumamplitude of the instantaneous signal frequency therein to therebypermit transmission in each sub-band in which an instantaneous signalfrequency occurs of only those interfering components having amplitudesnot substantially greater than the maximum amplitude of theinstantaneous signal frequency therein and com- .bining the energytransmitted through the varlous sub-bands and supplying the combinedenergy to a sound reproducing device.

7. A radio receiving system comprising means for separating the receivedenergy into a plurality of channels each of which covers a differentband of frequencies, means for instantaneously preventing transmissionof any energy through each channel in which there is no instantaneouscomponent of the received signal, means for preventing the transmissionthrough each channel in which a component of the received signal occursof any interfering component of greater amplitude than the instantaneoussignal frequency therein, and means for combining the energy transmittedthrough the various sub-bands and supplying the combined energy to asound reproducing device.

8. A radio receiving system comprising means for separating the receivedsignal energy into a plurality of channels each `of which covers adifferent band of frequencies, means connected in each channel forvariably limiting the amplitude of voltages which may be passedtherethrough, means operable by received signal frequency in thefrequency band of each channel to operate the limiting means of ysuchchannel to permit the sage through 'such channel of only thoseinterfering components having amplitude approximatelyequal to or lessthan the maximum amplitude of signal frequency instantaneously passedthrough the channel.

GEORGE DALLOS.

